The present invention generally relates to electrical dimmer and power control devices for discharge lamps and the like, and more particularly to those that deliver variable power in the form of high frequency, alternating current signals to a load.
Theoretically, electrical power may be varied by regulating any number of its basic parameters, but as a practical matter, some factors are more difficult to control than others owing to the characteristics of the electrical load. For example, a gas or electrical discharge lamp, such as the conventional fluorescent tube, poses a number of problems. The nonlinear conduction characteristic of the fluorescent tube makes voltage regulation, by itself, an impractical method of power control. The nonlinear conduction, or negative resistance, of the lamp requires a rather high voltage to turn the lamp on; once the arc is struck, however, the current must be limited or else the lamp might explode. Well known current limiting devices such as the conventional inductive ballast effectively maintain a constant operating level, but variable power controls that simply alter the current delivered to the fluorescent tube are effective only in a limited, medium brightness range. Too little current causes unwanted stroboscopic effects, shortened lamp life, and energy waste. Too much current, as previously indicated, may cause the lamp to explode.
Many devices vary the power delivered to a discharge lamp by altering the product of voltage times current. This mode of operation yields all of the liabilities of voltage or current regulation, although with some degree of improvement as a function of component design. Basically, the lamp is forced to operate at currents above or below an optimum value for creating a stable plasma discharge; furthermore, allowing voltage to change yields erratic firing.
Phase control allows alternating current to be applied to the lamp load only during specified parts of the a.c. waveform. A wide range of phase angles can be obtained, but the lower angles tend to cause flicker and inefficiency in fluorescent systems. Phase control devices also tend to be relatively expensive owing to the design of the inductors they employ.
High frequency drive systems for fluorescent lamps have existed for many years and beneficial side effects such as increased illumination efficiency have been frequently reported. Chopper systems, for example, are a class of high frequency regulators that sense an impending overcurrent event and shut themselves off until the current to the load drops below a critical value. Some choppers regulate current with a form of feedback-controlled, pulse width modulation. In another mode of regulation, the width of the pulses is more or less fixed, but the frequency of repetition varies. By changing the repetition rate, the power delivered to the load varies.
Power may also be regulated by changing the time factor rather than the current or voltage of the signal sent to the lamp load. The present burst control system utilizes this principle as well as the optimizing effects of high frequency signals. A value of voltage is selected which results in reliable firing of the discharge lamp or similar device; a value of current is chosen which provides a stable discharge; and a high frequency value is selected which increases the efficiency of the load's performance. Without more, these fixed parameters would theoretically result in constant power output. But if these fixed factors are applied intermittently for variable intervals of time, then the total power supplied to the load will change. Thus the present invention regulates its power output by providing high frequency, alternating current to the load for variable intermittent intervals, known as bursts.
The most relevant prior art known to the applicants are represented by U.S. Pat. Nos. 3,657,598 issued Apr. 18, 1972 to Nomura et al.; 3,927,349 issued Dec. 16, 1975 to Suhren et al.; and 4,087,722 issued May 2, 1978 to Hancock.
U.S. Pat. No. 3,657,598 issued to Nomura et al. discloses a control circuit which imparts time intervals longer than the turn-off time of its semiconductor switches to a pair of pulse sequences which drive the switches to conduction. While this time interval is functionally analogous to the dwell time provided by one form of the present invention, the Nomura control circuit is structurally quite dissimilar to the signal control means herein disclosed. Other components of the present power control apparatus bear little or no resemblance to the Nomura device.
U.S. Pat. No. 3,927,349 issued to Suhren et al. discloses a lamp dimmer which generates a pair of low frequency, variable pulse width synchronization signals and a relatively high frequency, zero-crossing control signal. The present adjustable power control develops pulse sequences somewhat analogous to Suhren's control and synchronization signals, but the present invention isolates a single, variable low frequency pulse sequence within a burst control device which, in turn, intermittently interrupts two high frequency phase-reversed pulse sequences for variable intervals before those high frequency signals reach first and second switches. In the Suhren device, however, two variable low frequency synchronization signals and a single high frequency control signal combine to form a final pair of pulses via two sets of NAND gates and pulse transformers. Also, the critical components of the two devices are structurally quite different.
U.S. Pat. No. 4,087,722 issued to Hancock discloses a lamp operating transformer and a flasher control that bear some resemblance to a preferred form of inductor configuration and the burst control device employed in the present adjustable power control; however, a number of dissimilarities are also readily apparent. First, the Hancock device dims the lamp load by varying the width of the individual pulses applied thereto. The present device, in contrast, generates pulses of fixed width and frequency and controls the power applied to the fluorescent load by varying the time interval between groups or bursts of these constant pulses. Secondly, in the lamp operating transformer of the Hancock device, the opposite ends of the primary winding apparently alternately receive the pulses of power, and the primary center tap is normally grounded. In contrast, the opposite ends of the transformer primary preferably employed in the present device alternately conduct a direct current signal received at the primary center tap to a pair of semiconductor switches. Third, the Hancock flasher control is intended to extinguish the lamp arc for variable intervals of time and appears to do so by grounding the switching devices so that no pulses reach the transformer primary. In contrast, our invention does not visibly extinguish the lamp arc; it intermittently interrupts portions of first and second pulse sequences for very short intervals of time, thereby allowing variable bursts of high frequency pulses to reach their respective switches. The Hancock device differs in structure and function from the present adjustable power control apparatus in other respects as well.